The present invention relates to a process for producing an electronic component, comprising the steps of providing a laminate having a layer construction of a conductive inorganic material layerxe2x80x94insulating layerxe2x80x94conductive inorganic material layer or a layer construction of conductive inorganic material layerxe2x80x94insulating layer, in which the insulating layer can be patterned by wet process, and wet etching the insulating layer to pattern the insulating layer, an electronic component per se produced by said process, and a suspension for a hard disk drive.
In recent years, rapid development of semiconductor technology has led to rapid progress of a reduction in size of semiconductor packages, the adoption of multipin, the adoption of fine pitch, minimization of electronic components and the like. That is, the semiconductor field has entered the so-called xe2x80x9cage of high density packaging.xe2x80x9d Regarding printed wiring boards, the above tendency has also led to a change from single side wiring to double side wiring and, in addition, the adoption of a multilayer structure and a thickness reduction (Iwata and Harazono, xe2x80x9cDenshi Zairyo (Electronic Material),xe2x80x9d 35 (10), 53 (1996)).
A pattern formation method used in the formation of wiring and circuits in such electronic components comprises etching a conductive inorganic material layer on a substrate in a layer construction of conductive inorganic material layer (mainly metal layer)xe2x80x94insulating layerxe2x80x94conductive inorganic material layer (mainly metal layer) with an acidic solution, such as a ferric chloride solution, to form wiring, then subjecting the insulating layer, for example, to dry etching such as plasma etching, or wet etching such as etching with hydrazine, to remove the insulating layer to form a desired shape for layer-to-layer continuity purposes (Japanese Patent Laid-Open No. 164084/1994), and connecting the wirings to each other, for example, through plating or electrically conductive paste. Another pattern formation method (Proceedings of the 7th Symposium of Japan Institute of Electronics Packaging, issued in 1999) comprises providing an insulating layer in a desired form using a photosensitive polyimide (Japanese Patent Laid-Open No. 168441/1992) or the like and then plating gaps to form wiring.
In recent years, for electrical appliances, particularly personal computers, there is a tendency toward lower prices. For members, components and the like used in them as well, there is a tendency toward lower prices. Therefore, how to produce electronic components at low cost is a great issue.
At the present time, rapid expansion of production of personal computers has led to increased production of hard disk drives incorporated in the personal computers. A component, in the hard disk drive, called a xe2x80x9csuspension,xe2x80x9d which supports a head for reading magnetism, is being shifted in its main products from one, wherein copper wiring is connected to a stainless steel plate spring, to one called a xe2x80x9cwireless suspensionxe2x80x9d comprising copper wiring which has been connected directly to a stainless steel plate spring, from the viewpoint of coping with the size reduction.
The above wireless suspension is mainly prepared using a three-layer material of conductive inorganic material layer (mainly metal layer)xe2x80x94insulating layerxe2x80x94conductive inorganic material layer (mainly metal layer). An example of the layer construction of the three-layer material is such that a copper alloy foil is stacked on one side of an insulating layer and a stainless steel foil is stacked on the other side of the insulating layer. The wireless suspension is a member, which is scanned on a disk being rotated at a high speed, and, thus, fine vibration is applied to the wireless suspension. For this reason, the adhesive strength of wiring (conductive inorganic material layer) to the insulating layer is very important, and the wireless suspension should satisfy strict specifications. Since hard disks are devices for recording information thereon, a high level of data read/write reliability is required and strict specifications should also be satisfied for the adhesive strength of wiring and useless materials, such as dust, and outgas produced from the suspension.
In the laminate such as the three-layer material used in electronic components, in general, in order to render the coefficient of thermal expansion of the conductive inorganic material layer and the coefficient of thermal expansion of the insulating layer identical to each other for preventing the warpage of the substrate, a low-expansion insulating layer, particularly a low-expansion polyimide, is in many cases included. In the laminate in which only a low-expansion polyimide is used in the insulating layer, polyimide films, for example, KAPTON (tradename; manufactured by Du Pont-Toray Co., Ltd.), Upilex (tradename; manufactured by Ube Industries, Ltd.), and APIKAL (tradename; manufactured by Kanegafuchi Chemical Ind. Co., Ltd.), are used as the low-expansion polyimide. A metal layer (mainly copper) is formed, for example, by sputtering or electroless plating on the surface of the low-expansion polyimide film, and, thereafter, the thickness of the conductor layer is increased by electroplating (hereinafter referred to as xe2x80x9ctype-I laminatexe2x80x9d). Another type of laminate is such that an insulator having a three-layer structure, wherein a layer of an adhesive (for example, an epoxy adhesive) other than polyimide is formed on the surface of a low-expansion polyimide, that is, having a layer construction of adhesive other than polyimidexe2x80x94low-expansion polyimidexe2x80x94adhesive other than polyimide, is provided and a conductor foil is adhered to the insulating layer by thermocompression bonding (hereinafter referred to as xe2x80x9ctype-II laminatexe2x80x9d). A further type of laminate is such that an insulator having a three-layer structure, wherein an adhesive polyimide layer is formed on the surface of a low-expansion polyimide, that is, having a layer construction of adhesive polyimidexe2x80x94low-expansion polyimidexe2x80x94adhesive polyimide, is provided and a conductor foil is adhered to the insulating layer by thermocompression bonding (hereinafter referred to as xe2x80x9ctype-III laminatexe2x80x9d).
In the type-I laminate, since the insulating layer is formed of a polyimide having a single composition, the warpage is less likely to occur. Further, a thin metal layer can be formed. Therefore, this is advantageous for the formation of fine wiring. On the other hand, in the type-II laminate and the type-III laminate, since the conductor layer is formed by thermocompression bonding, the conductor layer can be selected from various types. For example, the preparation of a laminate using a rolled copper foil, a stainless steel foil or the like is possible. The type-II laminate advantageously has good adhesion. For the type-III laminate, since the adhesive layer is formed of highly heat-resistant polyimide, the heat resistance is advantageously good. Further, both the type-II and type-III laminates have an additional advantage that the thickness of the metal layer can be increased.
Since spring properties are required of the wireless suspension, a stainless steel foil is in many cases used as the metal layer. An example of the laminate structure is copper foilxe2x80x94adhesive polyimidexe2x80x94low-expansion polyimidexe2x80x94adhesive polyimidexe2x80x94stainless steel. In the conventional wireless suspension, since the etching area of the insulating layer is large, instead of laser beam etching, plasma etching, which belongs to the same category of process, i.e., dry process, is mainly used for patterning of the insulating layer. In the plasma etching, however, the etching rate is low, and, thus, the time necessary for etching is long. Further, since sheet-by-sheet production is adopted, the productivity is low. Moreover, the apparatus for plasma etching is so expensive that the production cost is disadvantageously very high.
For the above reason, patterning of the insulating layer by a wet process, which is high in etching rate and thus is high in productivity and can realize low apparatus cost, has been desired in the art.
Also in electronic components, such as flexible printed boards or multilayer substrate, wherein a hole is formed by laser beams for providing continuity between layers in the multilayer substrate followed by pattern drawing in a mold into a desired form, when the wet etching technique is used, the step of hole formation and the step of pattern drawing can be simultaneously carried out. In addition, a fine shape, which cannot be formed in the mold, can be formed by wet etching. Therefore, patterning of the insulating layer by the wet process has also been desired in each field of electronic components.
For the type-II laminate, the use of an epoxy adhesive poses a problem that the solvent resistant is so high that wet etching cannot be carried out at all.
For the type-III laminate, due to a significant difference in etching properties between the adhesive polyimide layer and the low-expansion polyimide layer, the sectional form after etching is not sharp, and this makes it substantially difficult to prepare electronic components by wet etching.
For the type-I laminate, in some cases, wet etching is adopted. Since, however, the conductive inorganic material layer is formed by sputtering or the like, a metal is collided at a high speed against the surface of the polyimide. As a result, the metal bites into the surface layer, as well as into the inside of the polyimide layer. This somewhat denatures the polyimide in the surface layer. The adhesion between the insulating layer and the conductive inorganic material layer in the type-I laminate relies mainly upon a chemical bond or a bond derived from chemical interaction between the conductive inorganic material layer and the insulating layer. Therefore, the affinity of the conductive inorganic material layer for the insulating layer is high. This poses a problem that, when the type-I laminate is wet etched, the insulating layer in its denatured portion located at the interface of the conductive inorganic material layer and the insulating layer remains unetched, resulting in pattern defects.
On the other hand, in the laminate prepared by integrating the conductive inorganic material layer with the insulating layer by pressing, as compared with the chemical bond or chemical interaction, the anchor effect attained by concaves and convexes on the surface of the conductive inorganic material layer more greatly contributes to the adhesive strength. Therefore, an unfavorable phenomenon, wherein a portion to be etched remains unetched, is less likely to occur. As described above, in the production of the laminate by pressing, since the degree of freedom in the selection of the conductive inorganic material layer is large, products, which could not have been produced in the case of the formation of the conductive inorganic material layer by sputtering, can also be produced.
Further, unlike plasma etching, the wet etching does not require a high level of vacuum process. Therefore, a nonstop production line process can be easily applied by continuously feeding a continuous sheet of a laminate as a starting material, and, advantageously, the productivity can be significantly improved over the productivity of the plasma etching.
Polyimides generally have poor solubility in solvents. Since, however, they are decomposed by a hydrazine or alkali solution, various studies have hitherto been made on wet etching of polyimide films with a chemical liquid. For example, Japanese Patent Laid-Open No. 4577/1975 discloses a production process of a wiring structure using hydrazine and ammonia. Japanese Patent Laid-Open No. 103531/1983 discloses a method for etching a polyimide film with an inorganic basic aqueous solution. Japanese Patent Laid-open No. 65727/1982 discloses a method for etching a polyimide with an aliphatic diamine. Other methods for wet etching a polyimide disclosed up to now are such that a chemical liquid prepared by mixing water or an organic polar solvent with hydrazine/inorganic alkali/organic alkali/aliphatic amine (diamine) /aliphatic alcohol as a solvent is used (for example, Japanese Patent Laid-Open Nos. 74041/1983, 96632/1983, 101228/1991, 190610/1993, 202206/1993, and 157560/1995).
Hydrazine as a component for decomposing the polyimide, however, is highly toxic and thus is unsuitable for use in production process. For this reason, in proposals in recent years, in many cases, an etching solution comprising an inorganic basic aqueous solution and various additives added thereto is used.
Conventional methods for etching a polyimide film by wet etching to form a pattern include: a method wherein a metal is used in a pattern mask (Japanese Patent Laid-Open No. 283486/1993); a method wherein a solvent development-solvent separation-type negative-working liquid resist is used (Japanese Patent Laid-Open No. 301981/1993); and a method wherein a solvent development-solvent separation-type positive-working liquid resist is used (Japanese Patent Laid-Open Nos. 27464/1976, 49068/1978, 49068/1978, 65727/1982, and 74041/1983). These conventional methods for wet etching a polyimide film to form a pattern are effective in shorting the time necessary for patterning of the insulating layer.
The laminate using a polyimide as an insulating layer is in many cases thin and thus has low rigidity. Therefore, this laminate is disadvantageously inferior in handleability to conventional rigid glass epoxy substrates or the like. This is a serious limitation on process design. For this reason, if possible, a continuous production line by continuously feeding a continuous sheet is desired rather than sheet-by-sheet processing.
In the above method wherein the polyimide is etched using the metal layer as a pattern mask, however, after the metal layer is etched to form a final pattern shape of the insulating layer followed by etching of the insulating layer using the etched metal layer as a pattern mask, the metal layer should be further re-patterned by etching into a desired wiring shape. That is, in this method, the metal should be etched twice in total, and, in addition, the etching liquid comes into contact with the metal at the time of etching of the polyimide. This is causative of a deterioration in the metal layer.
Further, when a resist pattern is prepared using a solvent development-separation-type liquid resist, an organic solvent is necessary for the developing solution and the separating solution. Therefore, a load on the environment is large, and expenses for wastewater treatment are necessary. Moreover, when the solvent development-separation-type liquid resist is used, it is difficult to stably form a coating having even thickness for the following reason. Specifically, in the production of electronic components such as suspensions for hard disk drives, a liquid resist is coated onto a substrate having low rigidity, and the coating is dried. In this case, it is difficult to form a coating having even thickness. Patterning with high accuracy is required of electronic components such as suspensions for hard disk drives. In this case, when a resist film is formed by coating, very close control in the step of coating/drying is necessary for forming a coating having even thickness with high accuracy.
In continuously etching a continuous laminate having the three-layer structure, lamination by means of a flat press device in continuous lamination of the dry film is very disadvantageous. In this case, the use of a roll laminator is considered effective for coping with the continuous feed of the laminate sheet as the starting material. In the case of etching of the insulating layer by continuous feed of the laminate sheet, however, lamination of a dry film resist under atmospheric pressure onto the substrate with the metal being patterned poses a problem that, at the time of contact bonding, air bubbles are included into portions between the dry film resist and the laminate sheet, particularly in the edges of the patterned metal. Since air bubbles lead to etching failure, the inclusion of air bubbles should be prevented.
Basically, it is confirmed that, in the case of a basic aqueous solution developable-basic aqueous solution separable resist, when a polyimide etching liquid containing an inorganic alkali is used, the resist is disadvantageously separated from the laminate as a substrate by the action of an alkali component in the etching liquid. For this reason, it was thought that the realization of the production of electronic components by etching of the laminate using the above resist was difficult.
In techniques for wet etching of polyimides which are currently known in the art, a laminate comprising an insulating layer having one polyimide layer is mainly used. There are a few reports on examples of wet etching of a laminate of a plurality of polyimide layers (Japanese Patent Laid-Open No. 164084/1994). The reason for this is that, since the plurality of polyimide layers put on top of each other are different from each other in etching properties, a good sectional form cannot be disadvantageously provided by wet etching.
In the suspension for a hard disk, stainless steel as the spring material is an essential element. Therefore, the suspension is produced using a laminate produced by sandwiching a low-expansion polyimide film having on its both sides an adhesive resin layer between a conductor foil and a stainless steel foil and pressing the assembly, or a laminate produced by putting a plurality of polyimide layers on top of a stainless steel foil and further forming a conductor layer thereon by thermocompression bonding. Therefore, the production of the suspension faces the above various problems, and, for this reason, the preparation of a suspension by patterning of the polyimide layer by wet etching could not have been realized.
Accordingly, it is an object of the present invention to eliminate the above problems of the prior art and to provide a production process which, in the production of an electronic component by wet etching of an insulating layer in a continuous laminate of conductive inorganic material layerxe2x80x94insulating layerxe2x80x94conductive inorganic material layer or a continuous laminate of conductive inorganic material layerxe2x80x94insulating layer, wet etches a continuous sheet or a pseudo-continuous sheet at low cost without the use of any organic solvent, which poses a problem of waste treatment, in a continuous feed, continuous production line to produce an electronic component with high accuracy and with significantly increased productivity.
It is a subordinate object of the present invention to provide a production process of an electronic component which can realize single etching of the conductive inorganic material layer to prevent a deterioration in the conductive inorganic material layer, can eliminate the need to use any organic solvent, which imposes a large burden on the environment, as a chemical liquid for the development and separation of the resist, and enables the application of wet etching using a resist to an insulating layer having a single layer structure or a laminate structure of two or more insulation unit layers.
It is another subordinate object of the present invention to provide a production process of an electronic component that can provide a laminate which provides a good sectional form after wet etching with good etching accuracy, and to provide an electronic component and a suspension for a hard disk produced by said production process.
It is still another subordinate object of the present invention to enable wet etching of the laminate in the step of treatment using a dry film resist, that is, the step of development with an aqueous solution, particularly a basic aqueous solution and separation with a basic aqueous solution.
According to the present invention, there is provided a process for producing an electronic component, comprising the steps of: wet etching a laminate of conductive inorganic material layerxe2x80x94insulating layerxe2x80x94conductive inorganic material layer or a laminate of conductive inorganic material layerxe2x80x94insulating layer to pattern the conductive inorganic material layer; and then performing wet etching to pattern the insulating layer, wherein: the insulating layer in the laminate is wet etchable and has a single-layer structure or a laminate structure of two or more insulation unit layers; after the patterning of the conductive inorganic material layer in the laminate, the patterning of the insulating layer by wet etching is carried out in a continuous form in a continuous feed, continuous production line using a dry film resist; and, in patterning the insulating layer using the dry film, the dry film resist is laminated by roll pressing onto the laminate under a reduced pressure of not more than 80 KPa.
In the production process of an electronic component according to the present invention, in roll pressing the dry film resist onto the laminate under reduced pressure, this step is continuously carried out. Therefore, tension is always applied to the laminate, and this can solve the problem of warpage after the lamination.
In the production process of an electronic component according to the present invention, since the laminate as a continuous sheet is wet etched in a continuous form in a continuous feed, continuous production line, electronic components can be produced with high accuracy and with significantly enhanced productivity.
In a preferred embodiment of the production process of an electronic component according to the present invention, the dry film resist is developed with an aqueous solution, particularly a basic aqueous solution and is separated with an aqueous solution, particularly a basic aqueous solution. The development with the basic aqueous solution and the separation with the basic aqueous solution can solve the problem of the treatment of the used organic solvent.
As a result of studies on the resistance of various dry film resist to various etching liquids, it was found that, for basic aqueous solution development-basic aqueous solution separation-type dry film resists, when etching conditions, the thickness of dry film resists and the like are optimized as described later, some materials for dry films are resistant to etching liquids. Further, it was found that, likewise, aqueous lactic acid solution development-aqueous lactic acid solution separation-type dry film resists, which are dry film resists other than the basic aqueous solution development-basic aqueous solution separation-type dry film resists, are resistant to etching liquids.
In forming a pattern of the insulating layer in the laminate having the above construction in a sheet-by-sheet form, the lamination of a dry film resist onto the substrate, after the patterning of the metal, by means of a roll laminator causes the warpage of the laminate because the laminate per se is thin and has poor rigidity. The warpage of the laminate poses a problem that the misalignment with the mask at the time of exposure to the resist is significant. The wet etching of the insulating layer is characterized in that the dimensional accuracy of the patterning is higher than that in plasma etching which is an existing dry process. Therefore, in order to reproduce the insulating layer pattern in the laminate with high accuracy, the misalignment is a severe problem, and solving this problem is very important for establishing the wet etching process. According to the construction of the present invention, the laminate is treated in a continuous form in a continuous feed, continuous production line. Therefore, tension is applied to the continuous laminate, and, in the lamination of the dry film resist, the dry film resist is laminated by roll pressing under reduced pressure. This can prevent the occurrence of warpage.
The effect of preventing the inclusion of air bubbles at the time of the lamination of the dry film resist onto the laminate according to the present invention will be described. For the continuous laminate sheet having the above layer construction, even when the flatness is somewhat sacrificed, the feed of the laminate to a continuous production line to perform continuous treatment is important for enhancing the production efficiency. Therefore, for the continuous laminate sheet, after the etching of the inorganic material layer, when the insulating layer is wet etched using a dry film resist, since wiring of the conductive inorganic material layer is provided on the insulating layer, that is, since the wiring of the conductive inorganic material layer is the form of concaves and convexes, air bubbles are disadvantageously included in portions between the edges of the concaves or convexes and the dry film resist. In the present invention, as described above, in the lamination of the dry film resist, since the dry film resist is laminated by roll pressing under reduced pressure, the inclusion of air bubbles can be prevented.
Further, in the present invention, preferably, fine concaves and convexes are provided on the surface of the dry film resist. The formation of the air bubbles can be suppressed by laminating the dry film resist having concaves and convexes on its surface in such a manner that the concaves and convexes face the concave/convex side (that is, wiring side) of the conductive inorganic material layer. That is, the concaves and convexes form a place for escape of air bubbles to prevent the inclusion of air bubbles.
Further, in order to impart the above etching resistance of the dry film to the laminate, a method is preferably adopted wherein, after the laminate of the dry film resist is exposed and developed to form a pattern, treatment selected from ultraviolet light irradiation treatment, heat treatment, and a combination of ultraviolet light irradiation treatment with heat treatment is carried out to improve the resistance of the dry film resist to the etchant for the insulating layer.
Further, in order to develop an effect under the above conditions, the thickness of the insulating layer in the laminate is preferably 3 to 500 xcexcm. When the thickness of the insulating layer is larger than 500 xcexcm, the time necessary for etching is increased. In this case, since the dry film resist is dipped in the etching liquid for a long period of time, the shape of the dry film resist cannot be retained. On the other hand, when the thickness of the insulating layer is less than 3 xcexcm, insulation reliability is lowered.
Further, in order to impart the etching resistance to the laminate, the thickness of the dry film resist is preferably 1.1 to 5 times that of one conductive inorganic material layer in the laminate as the starting material.
The time necessary for the wet etching of the insulating layer is suitably not less than 10 sec and not more than 30 min, preferably not less than 10 sec and not more than 15 min, more preferably not less than 10 sec and not more than 5 min. When the time necessary for the wet etching of the insulating layer is not less than 30 min, the productivity is poor.
Furthermore, in order to impart the etching resistance to the laminate, the temperature of the insulating layer at the time of wet etching is preferably not less than 10xc2x0 C. and not more than 120xc2x0 C.
At the present time, the dry film resist, which is developable and separable with a basic aqueous solution, is most widely used and thus is inexpensive. Therefore, the number of types is large, and the range of selection is wide. Further, since a large number of apparatuses usable in the step of development and separation are also commercially available, a desired apparatus is easily available at a low price. In the case of an inorganic basic aqueous solution, the wastewater treatment is easy, and, thus, as compared with the solvent development-solvent separation-type liquid resist, the cost necessary for the whole process can be significantly reduced.
The laminate according to the present invention has a good sectional form after wet etching and can be etched with high accuracy and thus is useful particularly for a suspension for a hard disk drive.